Lithography is used in semiconductor fabrication via radiation to transfer images onto a substrate coated with a material reactive to the radiation. Radiation in the form of light, for example, ultraviolet light, may be directed onto a mask (i.e., a photomask) defining a pattern. After shining through or reflecting from a mask the light is projected through a series of optical lenses and/or mirrors that shrink the image. The reduced image is then projected onto the workpiece. The workpiece may, for example, be silicon or other semiconductor wafer covered with a radiation-sensitive photoresist. As the projected light hits the photoresist on the silicon wafer, it may alter the unmasked photoresist. Unaltered photoresist may then be chemically washed away, leaving patterned photoresist on portions of the wafer.
Immersion lithography (IL) is emerging as the technique of choice to print sub-100 nm photoresist structures using 193 nm radiation for semiconductor manufacturing. For 193 nm exposure wavelength pure water meets all the requirements for optimal semiconductor fabrication producing an index of refraction n≈1.44 and absorption of <5% at working distances of up to 6 mm. Water can also be compatible with photoresist and photolithographic lenses and degassed and decontaminated for a high level of purity.
A number of practical issues to implementing immersion lithography exist. The general process requires filling the gap between the last lens element of the exposure tool and the resist-coated substrate with ultra pure water. One approach has been to wholly or partially submerge the wafer stage, wafer and lens in a pool of water. The pool may be a recirculating pool or a stagnant pool. An issue with this approach is that submerging significant portions of multi-million dollar equipment requires significant re-engineering.
Another technique is to dispense water between the lens and the wafer with a nozzle. A suction port for liquid recovery may receive supplied liquid. Continuously maintaining a bubble-free even layer of water between the moving lens and wafer can be quite difficult using this technique, and larger topographical discontinuities, such as workpiece edges, complicate the engineering.
Even where immersion lithography has been shown to be a somewhat effective and simple enhancement technique to extend the limits of optical lithography, the contact between the immersion fluid and the resist could potentially lead to partial resist image degradation. One major concern associated to the introduction of immersion lithography at the manufacturing level is achieving adequate defectivity and overlay control.
All practiced methods used to manage the immersion fluid during the exposure step cannot perfectly contain the water within the scanner showerhead, and residual liquid in the form of droplets are expected to be left behind as the wafer scanning proceeds. Evaporation of residual water droplets from the immersion fluid during exposure can lead to uncontrolled cooling of the wafer surface, therefore leading to spatial pattern registration errors between different printed layers, which detrimentally affect the overlay budget. Simultaneously, the residual liquid is known to increase the post-exposure and post-development defectivity levels due to extraction and subsequent concentration of topcoat and resist components and/or environmental contaminants within the droplets upon evaporation.
One of the factors that controls the amount and size of water droplets that are able to break away from the liquid pool and escape the fluid containment system is the wettability of the topcoat or photoresist surface. A topcoat or resist material with high surface energy (hydrophilic) will generate a low contact angle between the immersion fluid (water) and the top surface of the imaging material. However, such enhanced wettability of the topcoat or resist surface can lead to water meniscus breakdown and residual droplet formation during the showerhead scanning process.
Contrarily, a topcoat or resist material with low surface energy (hydrophobic) will generate a high contact angle between the immersion fluid (water) and the top surface of the imaging material. Under high contact angle conditions the water pool integrity is maintained by the surface tension force acting on the surface of the liquid, and therefore the water pool can be more efficiently contained.
While certain hydrophobic materials are know, such as non-polar polymers, they are quite thick in application (e.g. 30 nm and above) and thus require further processing steps. In addition, they are expensive in and of themselves, while again adding expense to the manufacture due to the necessary additional processing steps. For example, a polymer may be added to an alcohol based photoresist topcoat, but the topcoat must be specific and the polymer layer must be baked, followed by exposure, a post-exposure bake step, a removal step and finally the developer step. The polymer layer may typically be more than 30-60 nm.
Thus, there remains a need for a more economical material and/or streamlined process that can provide a high contact angle between the top imaging surface of the photoresist layer and water.
These needs and many others are met by a process for coating the top of a photoresist layer with a fluorinated polymer. Other advantages of the present invention will become apparent from the following description and appended claims.